Opticianry is the professional practice of filling prescriptions for ophthalmic lenses and dispensing eyeglasses. When fitting progressive bifocals, the placement of the bifocal lens segment within the eyeglass lens should be such that the subject's gaze is centered through the bifocal lens when the subject is in a reading position, which is when the subject is holding his/her reading material about 300-500 millimeters (mm) away. Longstanding and current measuring techniques for determining the placement of bifocal lens segments within eyeglass lenses include physically measuring the distance between the optical axes of the patient's eyes, which is the distance between the centers of the pupils.
However, in a significant percent (e.g., about 25%) of bifocal wearers, the optical (pupillary) axis measurement produces an incorrect alignment because the optical (pupillary) axis of the eye is not always aligned with the visual axis of the eye. The visual axis is one's line of vision, which is a straight line that joins the fovea of the eye with a near fixation point. A consequence of using the optical (pupillary) axis measurement only for determining the placement of bifocal lens segments within eyeglass lenses is that the placement of the bifocal lens segments may not properly align with the patient's visual axis and, thus, the patient may experience blurriness and/or a noticeable degradation of the perceived binocular image. In particular, the viewing areas of progressive bifocal lenses are narrow and, thus, any slight misalignment is noticeable. As a result, the patient is dissatisfied with, for example, his/her progressive bifocal eyeglasses and further time and money is spent for return visits to the optician for follow-up corrective action.
For these reasons, a need exists for a visual axis and alignment measurement system for determining accurately the placement of bifocal lens segments within eyeglass lenses, in order to ensure that the resulting prescription for progressive bifocal corrective lenses, which have narrow viewing areas, is correct. In doing so, the time and money that is spent for return visits to the optician for follow-up corrective action because of poorly fitted progressive bifocals is reduced, as well as replacement of material.